I. Field of the Invention
The present invention is directed to an improvement in vehicle-mounted police radar warning receivers, and more particularly to such a receiver which can indicate the band in which the police radar is operating.
II. Description of the Prior Art
Police radar typically operates in one of two bands, e.g., either the X-band or the K-band of the frequency spectrum as discussed in U.S. Pat. No. 4,313,216, assigned to Cincinnati Microwave, Inc., the assignee herein. There are, generally, two types of police radar. One emits a continuous radar signal so long as the radar unit is turned on. The other emits a brief burst of radar signal when the police officer triggers the unit. This latter type is referred to as pulsed or instant-on radar. While transmitting, both continuous and pulsed radar transmit a signal which is at a fixed frequency within the selected band.
An electronic assembly referred to as a police radar warning receiver has been devised to detect the presence of police radar signals. The receiver is mountable in a vehicle, such as a passenger car or truck, motorcycle, boat or the like, which travels on land or water in areas subject to speed-monitoring radar surveillance by police, and functions to detect the presence of the police radar and provide the driver or user with an audible and/or visual indication that his speed is being checked by radar. The receiver is contained in a box-like housing which is set on the dash or clipped to the visor in the vehicle. Extending from the rear of the housing is a power cord which terminates in a plug adapted to be received in the cigarette lighter socket of the vehicle. The front panel of the receiver faces the driver and has various indicators and control knobs.
When police radar is operating within range of the radar warning receiver, the circuitry of the receiver is able to detect the presence thereof. The ESCORT and PASSPORT radar warning receivers, manufactured by the assignee herein, Cincinnati Microwave, Inc. of Cincinnati, Ohio, utilize a superheterodyne circuit for this purpose.
As explained in aforementioned U.S. Pat. Nos. 4,313,216, and 4,581,769, which is also assigned to the assignee herein, a superheterodyne circuit employs two local oscillators, one of which sweeps in frequency over a range of frequencies related to one or both radar bands. A first local oscillator may generate first and second oscillator signals which are mixed with signals present at the antenna. If the signals at the antenna are incoming police radar or other signals from either the X-band or the K-band, the oscillator signals mix therewith to produce signals in the passband of a first IF. The first oscillator signal may, for example, sweep from about 11.6 GHz to about 11.5 GHz while the second oscillator signal, which is preferably quasi harmonically related to the first, is sweeping from about 23.2 GHz to about 23.0 GHz. Police radar signals are typically transmitted in either the X-band at approximately 10.525 GHz or the K-band at approximately 24.150 GHz. Hence, X-band police radar signals are lower in frequency relative the first oscillator signals by a selected amount, while K-band police radar signals are higher in frequency relative the second oscillator signals by that selected amount. This relationship may be referred to as high side injection for X-band signals and low side injection for K-band signals and results in sweeping IF product signals which are centered at approximately 1.03 GHz upon receipt of either X- or K-band signals.
The sweeping IF product signals are then mixed with an oscillator signal from a second local oscillator, preferably at about 1.03 GHz, to produce signals in the passband of a second IF, which signals are then passed through a discriminator circuit to provide output pulses if a signal in either the X-band or the K-band was present at the antenna. As is understood, such a second heterodyning process will result in generation within the receiver of a pair of second IF signals, one the result of the sweeping first IF product crossing 1.04 GHz (the primary frequency) and the other the result of crossing 1.02 GHz (the image frequency). The discriminator will similarly generate a pair of pulses for each police radar signal received, one pulse related to the 1.04 GHz response (primary) as well as another pulse related to the 1.02 GHz image response (image).
As described in aforementioned U.S. Pat. No. 4,313,216, it is possible to determine whether the received radar signal is in the X-band or K-band based upon the timing or "spacing" between the primary and image pulses, e.g., the primary and image pulses resulting from receipt of X-band signals are spaced further apart than such pulses resulting from receipt of K-band signals. Some radar warning receivers may discriminate between X-band and K-band radar signals by using selected local oscillator frequencies to generate IF signals which are in one of two ranges depending upon the frequency of the received police radar signal. Thus, the passband of the IF may be increased and is therefore referred to herein as a spread IF approach. With the spread IF approach, the band may be determined by knowing in which of the two ranges the IF signal appears. The spread IF approach may also generate primary and image pulses although their spacing may no longer be relevant.
In order to improve sensitivity, and particularly with respect to superheterodyne receivers, to reduce noise, it has been known to provide an image reject filter in front of the second mixer to thereby filter out the image signal prior to the discriminator. However, this technique has not generally been available for police radar warning receivers because such a filter could preclude the possibility of band discrimination. For example, in the circuitry of the ESCORT and PASSPORT radar warning receivers, the "spacing" between primary and image pulses is utilized to determine the band as described in aforementioned U.S. Pat. No. 4,313,216, the disclosure of which is incorporated herein by reference. Filtering out the image signal (i.e., passing only the primary signal) would prevent generation of an image pulse thus inhibiting the ability to discriminate between bands by monitoring the spacing between primary and image pulses. In the spread IF approach, the image frequency is moving. Hence, an image reject filter, which has a fixed center frequency would not be useful. Some filters such as tunable YIG filter may provide a tracking image reject filter. Such a filter would be prohibitively complex and expensive to be employed in a consumer product such as a police radar warning receiver.
Additionally, many radar warning receivers adapted to indicate the band in which the police radar is operating will not always be able to determine whether the police radar signal is in the X- or the K-band, for example. This may be due, in some instances, to the presence of the image signal. Hence, it is desirable, if possible, to filter the image signal although this has not heretofore been advisable in a radar warning receiver as discussed. In any event, because it may be difficult to determine which band the police radar signal is in, some radar warning receivers may select one or the other band such as by default. This might lead to loss of intelligible information to the driver or user.